They aren’t of much use to players very often. It can be helpful to know where your pitches are sitting at, but other than that its only good for messing with your head. I only use them when charting my pre-season performances just getting a good idea where everything is.

Radar guns are useful for checking the differential between your pitches (e.g. fastball and change) and for measuring your progress when working on velocity.

You can tweak your off speed pitches to get the ideal differential. It seems 7-9 mph is a good difference. Less than that and it’s not really off speed, more than that and the pitch is easier to identify as off-speed.